1) Field of the Invention
The present invention relates to a rotational speed sensor comprising a rotatable ring, e.g. connectable to a bearing, having K magnetic pole pairs distributed angularly over the rotatable ring, K being an integer greater than one, and sensor means positioned relative to the rotatable ring such that a varying magnetic field is detected by the sensor means.
2) Description of the Related Art
Such a rotation sensor is known from German patent application DE-A-198 10 218.
A further rotation sensor is known from American patent U.S. Pat. No. 5,184,069, which describes a rotation sensor for detecting relative rotation between two components which are coupled by an anti-friction bearing. The rotation sensor comprises a tone ring with a layer of magnetic ink, the layer defining multiple magnetic poles, with the north and south poles being alternatively positioned at the circumference of the ring. The rotation sensor further comprises a transducer for detecting a varying magnetic field when the two components rotate with respect to each other.
However, the arrangement according to the prior art is susceptible to a number of error mechanisms. The layer having multiple magnetic poles is difficult to manufacture within very strict tolerances. The distance between north and south oriented magnetic poles on the layer is not always constant over the entire layer circumference. This causes the detected magnetic field to have anomalies when the disc rotates, caused by the phase errors, also indicated by the term jitter.
Further problems occur when the layer on which the magnetic poles are arranged is not making a perfect circular motion. This may be caused by radial movement of the layer with respect to the magnetic sensor, and causes further errors in the sensor output signal.
Also, external magnetic fields may influence the signal generated by the magnetic sensor.